1. Field of the Invention
The present invention relates to a process for preparing a soft magnetic film of Permalloy for use as the magnetic core of a thin film magnetic head or the like, and more particularly to a technique for preparing a soft magnetic film of Permalloy in an inert gas atmosphere by the multielectrode sputtering method.
2. Description of the Prior Art
A soft magnetic film of Permalloy is used for the magnetic core of a thin film magnetic head which is employed as the magnetic recording-reproduction means in the field of audio, video and other magnetic data processing. Generally, the Permalloy film has heretofore been prepared by electro-plating.
However, with the development of semiconductor techniques in recent years, the sputtering method has been positively discussed for use in preparing films. Although the plating method requires an electrically conductive ground layer for plating and use of an agent for mitigating the stress in the film, the sputtering method forms compact films without necessitating these means. Furthermore, the sputtering method forms a film of high purity free from the above agent or other additives on a ground layer for which a wide variety of materials are usable.
The radio-frequency diode sputtering method is usually employed for forming films by sputtering. With this method, the input power is controlled for setting the electrical condition, so that the target voltage and the target current for sputtering can not be controlled independently of each other. On the other hand, it is already known that when Permalloy films are formed in argon or like inert gas pressure by the sputtering method, the magnetic properties of the film vary with high sensitivity according to the conditions, such as argon gas pressure and target voltage, set for sputtering. Thus, the radio-frequency diode sputtering method encounters difficulties in preparing Permalloy films of specified excellent magnetic properties with good reproducibility.
To overcome these drawbacks, a multielectrode sputtering method has already been proposed in which the target voltage and the target current are independently controllable. Permalloy films having outstanding magnetic properties and uniaxial anisotropy can be prepared by this method with good reproducibility, by controlling the pressure of inert gas and setting the target voltage at a value below the threshold voltage where the coercive force of the Permalloy film increases abruptly.
It has also been proposed to alter the magnetic properties and crystalline structure of the Permalloy film to be formed by applying a negative bias voltage to the substrate during sputtering. (C. S. Cargill III, S. R. Herd, W. E. Krull and K. Y. Ahn, "Structure-Sensitive Magnetic Properties of R.F. Sputtered Ni-Fe Films," IEEE Trans. Magn., Vol. MAG-15, no. 6, pp. 1821-1823, November, 1979; Y. Hoshi, M. Kojima, M. Naoe and S. Yamanaka, "Preparation of Permalloy Films by Using Targets Facing Type of High Rate and Low Temperature Sputtering Method," Trans. IECE Japan, Vol. J65-C, no. 10, pp. 783-790, October, 1982, in Japanese)
The conventional methods, including those described above, for preparing Permalloy films form the film on a flat substrate. However, when the Permalloy film is to be actually used as the magnetic core of a thin film magnetic head or the like, the magnetic core of Permalloy film, as indicated at 5 in FIG. 1, is prepared using a magnetic substrate 1 of Ni-Zn ferrite, Mn-Zn ferrite or the like, or a nonmagnetic substrate coated with Permalloy or Sendust, after forming an insulating layer 2 of SiO.sub.2 or the like over the substrate, providing a winding conductor 3 of Cu, Al or the like on the layer 2 and further forming a second insulating layer 4 of SiO.sub.2 or the like over the conductor 3. Accordingly, the ground layer over which the Permalloy film is formed is not planar but is a rugged surface having stepped portions. For example, the Permalloy film is formed also over slopes as indicated at a and b in FIG. 1. It is well known that when the Permalloy film is formed over such slopes by vacuum evaporation or sputtering, oblique impingement of the particles to be deposited on the substrate impairs the magnetic properties of the Permalloy film. This entails the problem that the magnetic core constituting the magnetic circuit of the magnetic head locally has increased magnetic resistance, which reduces the efficiency of the magnetic head.